add instruction "ld hl, sp+nn"

The previous "ld hl, [sp+nn]" is incorrect, but remains for compatibility.
Improve rgbasm performance
Merge branch 'update-legacy-instructions' of https://github.com/yenatch/rgbds
Make _NARG work
Remove __LINE__
Cleaned up lexer
- separated the lexer into multiple functions so it is more readable
- fixed issue with long label names in macro arguments
- added error checking code to prevent buffer overflows
rgblink: Don't allocate unnecessary buffer. Delete unused flag.
rgbasm: Don't allocate an unnecessary buffer.
rgbasm: Fix TOCTOU and reduce buffering.
Unclutter Makefile: use $Q instead of ${Q}.
Wrap Makefile to 80 characters.
Improve POSIX compliance in the Makefile.

Set .POSIX and don't use the += operator.
rgbasm: Avoid unnecessary filename copying.
Remove dead stores.
Now that we replace missing libc functions, switch back to err().
Remove rgblib.

I have never used it and it's probably been broken for years.
Update license.
Fix MinGW target.
Revert 97d431d1f4ad404e282e3781bd195be3f053734d; it breaks things.
Separate errors that shouldn't have been combined in the first place.